1. Field of the Invention:
This invention relates to a method and apparatus for selectively carrying first and second longitudinal loads in a tubing string in a subterranean well, with the apparatus selectively separating and severing the tubing string when the second weight load is exceeded.
2. Description of the Prior Art:
In the production of petroleum fluids, such as oil and/or gas, from wells, it has been the practice to provide automatically closing shutoff or safety valves which are located downhole and are held open by control fluid pressure. The valves automatically close when control fluid pressure is reduced. When damage occurs to the control fluid system at the wellhead or on an offshore platform, the reduction in control fluid pressure results in the valve closing. Such valves are employed below the wellhead and, in the case of offshore wells, the valves are installed below the mud line at such depth as may be desired or established by regulation. In the event of damage of a well caused by shifting earth or subsidence, or wellhead catastrophe, the well can be shut-in to avoid loss of valuable well fluids into the water, and also to avoid contamination of the water and the shore.
Since, from time to time, it is necessary to perform various remedial operations through the tubing string, it is preferred that the safety valves be easily removable from the well for service or repair. Accordingly, commercially available safety or shutoff valves have been produced which have been run into the well casing on production tubing and landed in a tubing hanger which supports the greater weight of the downwardly extending production tubing strings. Typically, such a tubing string has been run into the well casing on a setting tool to a desired depth and, in the case of an offshore well, it is to a prescribed depth below the mud line. In such an apparatus the tubing hanger is anchored in the well casing and the setting tool is released from the tubing hanger and removed from the well. The tubing hanger provides a seat for the safety or shutoff valve assembly which is run into the well on an upward extension of the production tubing and landed in the tubing hanger, subsequent to the setting of the hanger and retrieval of the hanger setting tool.
Typical of such prior art apparatus is as disclosed in U.S. Pat. No. 3,771,603, issued Nov. 3, 1973, entitled "Dual Safety Valve Method and Apparatus", to Talmadge L. Crowe. The necessity of two trips into the hole with work strings and/or other means to first carry an anchoringly set the tubing hanger and thereafter land the conduits containing the safety valves therein, is an economic problem. Considerable rig time is expended in running a first work string and/or other means for anchoring the hanger, retrieving the work string, and thereafter running the production tubing containing the safety valve or valves into sealing engagement with the hanger.
The use of a shear-out safety joint incorporable within the tubing string which will support the loads imposed by the tubing extending therebelow would alleviate this provblem. Shear-out safety joints are intended, however, to fail when subjected to longitudinal loads generally less than the weight of the tubing.
U.S. Pat. application, Ser. No. 036,910, filed May 7, 1979, discloses a shear-out safety joint used with a tubing hanger. That safety joint has a wireline retrievable interior bridge. A double collet secures the bridge and the joint together so that a weight load in excess of the desired shear-out load which can be carried across the tool will result in separation of the upper and lower tubing string. The double collet member has outwardly facing dogs which engage cooperating recesses on both the upper and lower relatively movable sections of the shear-out safety joint. The collet is removable by wireline techniques, thus leaving the two telescoping members secured only by shear pins capable of carrying a prescribed shear load. After the collet is removed and this prescribed shear load is exceeded, the shear-out safety joint will separate.